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The measurement of radio frequency complex permeability of thin round wires

Stuart, Thomas (Thomas Edward Walter) (2003-04)

Thesis (PhD)--University of Stellenbosch, 2003.

Thesis

ENGLISH ABSTRACT: This thesis is concerned with the measurement of the complex permeability of
thin round wires at radio frequencies. This is of interest as such wires are used
in various applications, such as absorbing chaff. Iron and nickel alloys are also
used for their good tensile properties but have an undesired electromagnetic effect
which needs to be characterised. Although little work has been done in this field
in recent decades it remains a relevant problem. In this thesis the advantages of
accurate wide-band measurements performed by automatic network analysers are
applied to the field.
The measurement system is a closed coaxial transmission line with a short
circuit termination. The centre conductor is the wire of interest. The surface
impedance of the wire is related to complex permeability and is measured using
low-loss transmission line approximations applied to half-wavelength resonances.
The loss associated with complex permeability is separated from conductivity by
a D.C. conductivity measurement.
A full wave analysis of the coaxial mode was performed and compared to
measured values. The maximum error of the propagation constant was found to
be 31% at the highest frequencies and was primarily due to length uncertainties.
By varying parameters expected error bands around the measured permeability
were found. These bands are of the order 1 and demonstrate that the system is
sufficiently robust.
The measurement of the permeability of two non-magnetic wires was performed
and a relative permeability of 1 was found, demonstrating the correct
working of the system. A steel wire was measured and compared to measurements
found in literature. The permeability dropped as frequency rose as was expected,
and an acceptable comparison to other measurements was made as there
is no verification standard.
Thus a simple measurement system that takes advantage of calibrated automatic
network analyser measurements has been developed and demonstrated to
work with sufficient accuracy.